Perf will depend on this. We don't have a way to express that dependency
yet, but at least we'll build the library before building perf when you do
a make apps-install.

Note that elfutils isn't added to our codebase - just a Makefile that will
build it and drop it in your sysroot.

You'll need to make fill-kfs after building this, just like any other
shared library (e.g. glibc).

Oh, this also accidentally gave us readelf and a bunch of other tools that
should run natively. Right now, they are installed to the sysroot/usr/bin
instead of kern/kfs/bin. We probably don't want an extra 11 MB for tools
we won't use yet.

I attempted to have make depend on $(build-dir)/Makefile instead of
config. This would work for a clean repo, but later we'd have issues.
If we had successfully configured, then did something that caused a
reconfig, make would get confused. It would see that
$(build-dir)/Makefile existed, then it would rebuild build-dir, removing
the Makefile. It would skip the config step, since it thought that
build-dir was older than Makefile, for whatever reason. i.e.

Prerequisite `elfutils-0.164' is older than target `elfutils-0.164/Makefile'.
No need to remake target `elfutils-0.164/Makefile'.

If someone has a nice fix, then we can change it. Until then, every
time we make, we'll also have to configure.